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The Double-Slit Experiment In the double-slit experiment a beam of light is passed through a piece of card with a slit in it and onto another card with two adjacent slits and then the resulting pattern is projected onto a screen. The pattern has a striped appearance due to what is called the interference effect. This is where the light waves that have passed through one slit interfere with light passing through the other slit. Light waves are similar to water waves in that they are made up of a movement that goes up and down. If two waves mix together they ‘interfere’ with each other. If the waves are ‘in phase’ (that is, the two waves are moving up an down at the same time) the movement is added together to produce one giant wave. If the two waves are ‘out of phase’ (that is, when one wave is reaching a peak the other is reaching a trough) they cancel each other out and the result is no wave at all. In the case of the slit experiment, the light travels a slightly different distance from each slit and depending where it hits the screen determines whether the wave is in phase or out of phase at that point. If instead of sending a wave through the slits we fired marbles, there would simply be two stripes on the screen: one for each slit, as the marbles cannot interfere with anything. Light consists of photons, a photon is a unit of light and a beam of light is a stream of photons coming in waves. So the double-slit experiment can be repeated with a small variation. Instead of sending a continuous beam of light through each slit we send one photon at a time. There should be no interference pattern as the photon has nothing to interfere with. However, we do observe an interference pattern. In a refinement of the experiment a detector is set up to determine which of the two slits the photon passes through and in this case there is no interference pattern. The act of observing the experiment has changed the result. When we do not observe which slit the photon is going through it acts as wave. When we do observe it, it acts as a particle. This act of seeing a particle rather than a wave is referred to as ‘collapsing the wave’. The wave is considered to be a wave of possible routes and when we single one out we collapse this. That the act of observing the result changes it creates a problem for physics as the whole basis of experimental physics demands repeatability. It also raises issues regarding the nature of reality and the interaction between mind and matter. In this short article I can only summarise what is a very complex issue but, in short, there are three main theories to attempt to explain this: The universe has a relationship with the mind. Some early experimenters (eg Fritjof Capra) noticed an intriguing relationship between quantum physics and Eastern Mysticism. Thus in this theory the act of a conscious mind viewing the result has affected the outcome. There are an infinite number of universes and every possibility is resolved into one universe or another. We happen to live in a universe where the possibilities resolved in the particular way they did. There is nothing mysterious going on but there are particles that are able to travel faster than light and therefore appear to go back in time. Once the wave has been collapsed the particles go back in time to resolve the outcome. In the first case the relationship between the objective and subject universe becomes blurred. It opens up possibilities of affecting the universe through the mind. I’ve written on this previously. One experimenter remarked that if the human race were to die out, the moon would cease to exist eventually as the small particle effects permeated through the whole moon. It may take millions of years though. In the theory of multiple worlds we have a mathematical sleight of hand to attempt to resolve the problem. The number of parallel worlds is mind-boggling. Presumably every time a particle interacts with another there is new complete universe created. There is no evidence for this infinite number of universes whatsoever. It also raises the question of the relationship between ourselves in this world and the selves in the parallel worlds. The third theory opens up interesting possibilities. There is no reason, theoretically, why faster than light particles can’t exist. Einstein’s theory of relativity simply states that a particle cannot pass through the light speed barrier. A particle can be created that travels faster than light though. A definition of intelligence is that intelligence denotes the ability to predict the outcome of an action. So the more intelligent an organism or person is, the better they are at taking an action to produce a desired outcome. Now, if we have particles that can travel back in time to ensure that a particular outcome is produced, these particles are the agents of infinite intelligence. The outcome must be produced with an absolute certainty. This is surely intelligence of a God-like quality. It opens up the question of how mind can interact with matter (for example in prayer) to bring about miracles.